1. Field of the Invention
The present invention relates to a solid state imaging device provided with a micro lens and a method for manufacturing the device.
2. Description of the Prior Art
Generally, a solid state imaging device has on its semiconductor substrate both a photodetecting portion and an electric charge transfer portion for transferring electric charges in the photodetecting portion. Therefore, a photodetecting side surface of the semiconductor substrate cannot be used entirely as a photodetecting portion. For the above reason, there is a recent scheme to optically refract light which is about to be incident on the electric charge transfer portion and make the light be incident on the photodetecting portion to thereby increase the light incident on the photodetecting portion and thereby enhance the sensitivity of a solid state imaging device.
A method for manufacturing such a solid state imaging device is shown in FIGS. 2(a) through 2(d) (Japanese Patent Publication No. SHO 60-59752). First, in a manner as shown in FIG. 2(a), there is formed a flattening layer 4 which covers a photodetecting portion 2 and an electric charge transfer portion 3 formed on a semiconductor substrate 1 to flatten the surface. Then, in a manner as shown in FIG. 2(b), a thermosoftening resin having a photosensitivity is coated on the flattening layer 4 to form a thermosoftening resin layer 6, and an upper surface of the thermosoftening resin layer 6 is covered with a photomask 11 to perform photo-etching. By so doing, as shown in FIG. 2(c), each portion of the thermosoftening resin layer 6 to serve as a boundary between micro lenses above the electric charge transfer portion 3 is removed to form rectangular parallelepiped thermosoftening resin blocks 61. Subsequently, in a manner as shown in FIG. 2(d), the thermosoftening resin block 61 is heated so that corner portions of the block 61 are softened to be thermally transformed to thereby form an approximately hemispheric micro lens 62.
Another manufacturing method is shown in FIGS. 3(a) through 3(e) (Japanese Patent Laid-Open Publication No. SHO 60-60757). First, in a manner as shown in FIG. 3(a), there is formed a flattening layer 4 which covers a photodetecting portion 2 and an electric charge transfer portion 3 formed on a semiconductor substrate 1 to flatten the surface. Then, in a manner as shown in FIG. 3(b), an optically high refractive index material layer 16 having a high optical refractive index is formed on the flattening layer 4. Then, in a manner as shown in FIG. 3(c), a photoresist layer 12 is formed on the high refractive index material layer 16, and each region of the photoresist layer 12 corresponding to the electric charge transfer portion 3 is removed by photo-etching to form a block 121 composed of a rectangular parallelepiped photoresist corresponding to the photodetecting portion 2. Subsequently, in a manner as shown in FIG. 3(d), the block 121 is heated so that corner portions of the block 121 are softened to be thermally transformed to form an approximately hemispheric block 122 composed of the photoresist. Then the high refractive index material layer 16 is subjected to anisotropic etching with the approximately hemispheric block 122 which is composed of the photoresist used as a mask to form an approximately hemispheric micro lens 161 as shown in FIG. 3(e).
According to either of the above-mentioned two methods, the rectangular parallelepiped thermosoftening resin block 61 or the rectangular parallelepiped photoresist block 121 is heated to be thermally transformed into approximately hemispheric micro lens 62 or block 122, and therefore the following problem takes place. In detail, according to the former manufacturing method, when the rectangular parallelepiped block 61 composed of a thermosoftening resin as shown in FIG. 4(a) is heated to be thermally transformed into a micro lens, corners of the bottom surface of the resulting micro lens 62 are slightly rounded by the surface tension of the thermosoftening resin. For the above reason, as shown in FIG. 4(b), the area of an ineffective region A (indicated by hatching) where incident light is not used effectively becomes greater than an idealistic ineffective region B as shown in FIG. 4(c) to have a reduced sensitivity. The above-mentioned phenomenon becomes more significant according as the micro lens is made finer because the influence of the surface tension of the resin increases.
In order to increase the light collection quantity of the micro lens 62, the gaps between a plurality of micro lenses 62 are required to be reduced as far as possible. However, there is a limitation in thermal fluidity of the thermosoftening resin and post-etching control of line width, heating temperature, and the like, and therefore it is difficult to reduce the gap between adjacent micro lenses 62 below 1.0 .mu.m. The above fact also causes the problem that the area ratio of the gap regions between micro lenses with respect to the entire photodetecting surface increases to have a reduced sensitivity according as the micro lens of the solid state imaging device is made finer.
According to the latter manufacturing method, the rectangular parallelepiped block 121 composed of the photoresist is also heated to be thermally transformed into the approximately hemispheric block 122 composed of the photoresist. Therefore, the corners of the bottom surface of the approximately hemispheric block 122 are slightly rounded. For the above reason, the corners of the bottom surface of the resulting micro lens 161 are slightly rounded to cause the same problem as that of the former manufacturing method.